Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer using an electrophotographic printing method.
Description of the Related Art
Up to now, as a charging unit configured to charge an electrophotographic photosensitive member (hereinafter referred to as photosensitive member) serving as an image bearing member uniformly at a predetermined polarity and potential, a contact charging device is put into practical use for an image forming apparatus using an electrophotographic printing method because of having advantages such as low ozone and low power. Contact charging devices are of the type that charges a photosensitive member by applying a voltage to a charging member which is brought into contact with the photosensitive member. Contact charging devices which employ a roller charging method, where a charging roller (conductive roller) serves as a charging member, are particularly preferred in terms of charging stability, and are widely used.
The roller charging method is divided into an AC charging method, which applies a superimposed voltage generated from a direct current (DC) voltage and an alternating current (AC) voltage (oscillation voltage) to the charging roller as charging bias, and a DC charging method, which applies only a DC voltage as charging bias. The DC charging method is in use in recent years from the viewpoint of saving cost and space.
With the DC charging method, however, it is difficult to even out uneven potential of the photosensitive member after transfer (hereinafter referred to as transfer memory), and transfer memory is visible as an image in some cases.
Transfer memory is a phenomenon originating from unevenness in electric potential which is caused on the photosensitive member after the transfer of a toner image from the photosensitive member to a recording medium (recording material or secondary transfer member) by a difference in the amount of transfer current flowing to the photosensitive member surface between a portion where toner is present and a portion where toner is not present. The uneven potential on the photosensitive member cannot be evened out satisfactorily in a charging step of the subsequent image forming cycle, and is consequently visible on the formed image.
A conventional solution is optical residual charge elimination (whole surface erasure exposure) performed by a memory erasure unit in order to even out once the potential on the surface of the photosensitive member after transfer. However, providing a memory erasure unit causes an increase in the size and cost of the image forming apparatus.
As a way to reduce transfer memory without adding a memory erasure unit, background area exposure is known (see Japanese Patent Application Laid-Open No. 2008-8991). In background area exposure, an exposure unit exposes a printing portion on the photosensitive member where a toner image is formed to light after the photosensitive member is charged to a predetermined potential in the charging step, and concurrently exposes a non-printing portion where no toner image is formed to a small light intensity.
Photosensitive members, however, gradually succumb to light-induced fatigue by exposure. The decrease in the photosensitivity of the photosensitive member due to light-induced fatigue therefore needs to be taken into account in a unit which keeps the photosensitive member surface exposed to light as in Japanese Patent Application Laid-Open No. 2008-8991. With the advance in the prolonging of product life and the enhancement of image quality in recent years, as well as ever-greater user diversity, long-term stable performance is demanded of photosensitive members. In order to accomplish further prolonging of product life, it is important to reduce light-induced fatigue of the photosensitive member as much as possible and thereby reduce a decrease in sensitivity.